fix(plugin): use directory form for skills (auto-discover)

The Claude Code plugin schema accepts string|array for the `skills`
field; passing a directory path auto-discovers every `<name>/SKILL.md`
beneath it (per https://code.claude.com/docs/en/plugins-reference,
which lists `"./custom/skills/"` as a valid example).

Switching `skills` from a 21-element array to `"./skills"` keeps the
fix for #112 (skills are still explicitly declared, so discovery is
not suppressed) while removing the maintenance burden — adding a new
skill no longer requires also editing this manifest.

`agents` stays as an explicit array because `./agents/` contains
`README.md`, which would be picked up as a malformed agent if the
directory form were used here.

.claude-plugin/plugin.json

@@ -9,29 +9,7 @@
   "repository": "https://github.com/addyosmani/agent-skills",
   "license": "MIT",
   "commands": "./.claude/commands",
-  "skills": [
-    "./skills/api-and-interface-design",
-    "./skills/browser-testing-with-devtools",
-    "./skills/ci-cd-and-automation",
-    "./skills/code-review-and-quality",
-    "./skills/code-simplification",
-    "./skills/context-engineering",
-    "./skills/debugging-and-error-recovery",
-    "./skills/deprecation-and-migration",
-    "./skills/documentation-and-adrs",
-    "./skills/frontend-ui-engineering",
-    "./skills/git-workflow-and-versioning",
-    "./skills/idea-refine",
-    "./skills/incremental-implementation",
-    "./skills/performance-optimization",
-    "./skills/planning-and-task-breakdown",
-    "./skills/security-and-hardening",
-    "./skills/shipping-and-launch",
-    "./skills/source-driven-development",
-    "./skills/spec-driven-development",
-    "./skills/test-driven-development",
-    "./skills/using-agent-skills"
-  ],
+  "skills": "./skills",
   "agents": [
     "./agents/code-reviewer.md",
     "./agents/security-auditor.md",